Major Description
What changes were proposed in this pull request?
Add new functions DATE_FROM_UNIX_DATE and UNIX_DATE for conversion between Date type and Numeric types.
Why are the changes needed?
1. Explicit conversion between Date type and Numeric types is disallowed in ANSI mode. We need to provide new functions for users to complete the conversion.
2. We have introduced new functions from Bigquery for conversion between Timestamp type and Numeric types: TIMESTAMP_SECONDS, TIMESTAMP_MILLIS, TIMESTAMP_MICROS , UNIX_SECONDS, UNIX_MILLIS, and UNIX_MICROS. It makes sense to add functions for conversion between Date type and Numeric types as well.
